a) Field of the Invention
The present invention relates to an electronic circuit using a memory, and more particularly to a musical tone signal control system using a central processing unit (CPU) and a digital signal processor (DSP).
b) Description of the Related Art
A CPU is commonly used in an electronic musical instrument to generate and control a musical tone signal. CPU executes a program stored in a read only memory (ROM) by using a random access memory (RAM) as a working memory such as registers and makes a tone generator generate a tone signal.
Because of recent demands for sophisticated and versatile musical toes of an electronic musical instrument, the amount of signals to be processed by a tone signal control system has increased and it has been desired to speed up signal processing. In order to meet such requirements, DSP is prevailing for adding musical effects to a tone signal, particularly reverb and the like.
FIG. 7 shows an example of a circuit of an electronic musical instrument according to a prior art. Referring to FIG. 7, a CPU 53, memories 61 such as a ROM and a RAM, a tone generator 54, and a DSP 55 are connected to a CPU bus 51. A keyboard 65, tone color switches 67, and the like are also connected to the CPU bus 51 via an interface (I/F) 64.
Another memory 63 is connected to DSP 55 via a dedicated DSP bus 62. An output of DSP 55 is supplied to a sound system 57 including an amplifier, a loudspeaker, and the like, via a digital-analog converter (DAC) 56.
As a player manipulates the keyboard 65 and performs music, a music performance signal is sent via I/F 64 to CPU 53. In accordance with a program stored in the memory 61 and by using registers therein, CPU 53 generates tone parameters for generating a played tone signal and sends them to the tone generator 54. A tone signal generated by the tone generator 54 is supplied to DSP 55 so as to add musical effects such as reverb.
DSP 55 executes a predetermined music performance process by using the memory 63 such as a RAM, and sends the tone signal added with musical effects to DAC 56. DAC 56 converts the tone signal into an analog tone signal which is supplied to the sound system 57 to produce music sounds.
When any one of the tone color switches 57 is activated, a switching signal is sent via I/F 64 to CPU 53 which refers to the memory 61 and changes parameters or the like for the tone generator 54.
Recent improvement of integration of semiconductor devices has made possible to assemble both CPU and DSP on a single chip. With the advent of a single chip CPU and DSP, it is supposed that electronic circuits such as shown in FIG. 7 will prevail more and more.
Memories are often formed on a different chip from a single chip CPU and DSP. It is necessary to provide a CPU bus between CPU and memories and a DSP bus between DSP and memories. As a result, use of such a single chip CPU and DSP greatly increases the number of pins of a semiconductor integrated circuit.
It is often that an access frequency of DSP to a memory is much smaller than a maximum access frequency. In other words, a memory for DSP becomes idle during a long time. However, DSP is required to strictly synchronize with a DAC cycle of DAC connected to DSP, and no wait is permitted.